History
The introduction of programmable logic devices (PLD) was a true revolution in the hardware design world. It enabled engineers to shrink circuits requiring several devices onto a single device thus simplifying their designs while saving space and power. Traditionally, PLDs have been used in combinational circuits such as address decoders as well as sequential circuits such as bus arbitration schemes. During the last few years. advances and improvements in PLD architectures enabled the devices to grow more complex while addressing the never-ending quest for higher density and faster speeds.
A certain class of FPLDs has been developed which are known as field programmable logic devices, or FPLDs for short. FPLDs chips that can be programmed or modified by the chip users or purchasers with the aid of readily available programming equipment to change the logic circuitry as desired.
For example, one common usage of PLDs is to provide two types of output signals. A first type of output signal is known as registered or stored signals. This signal is enabled through the use of registers positioned in output structures known as macrocells. A second type of output signal is known as combinatorial or non-registered signals. This second type of signal is accomplished by not sending the logic signals, entering the macrocell, through registers; thus being unstored signals or combinatorial logic signals.